1. Field of the Invention
The present invention relates generally to an imaging method and system for providing acoustic images of a surface or boundary and more particularly, but not exclusively, to acoustic imaging of a surface or boundary within a confined space. Also provided is an acoustic probe and a transmit/receive sensor array configuration for implementing the imaging method and system.
2. Description of the Related Art
There are many applications in which the image of a surface requires inspection within a confined space. For example, in the field of oil extraction, an oil extraction pipe is housed concentrically within a larger diameter steel pipe, which acts as a casing for isolating the well bore and the extraction pipe, from potentially dangerous high-pressure zones. The steel casing pipe is cemented in place within the well bore and has a diameter of between 5 to 30 inches (13-76 cm), the range of size accommodating different diameters of well bore which may decrease in several stages as the well bore passes deeper below the surface. The oil extraction pipe typically has a diameter of 4 inches (10.16 cm).
It is necessary to check the integrity of the oil extraction pipe structure by inspecting the inner surface of the pipe to investigate fractures, damage or other effects, such as the build up of sediment, which may hinder or prevent the extraction of oil along the pipe. As the pipe is only 4 inches in diameter the devices adopted to carry out the inspection are restricted in size. Furthermore, other technical constraints are encountered related to the application. For example, if an optical probe, such as endoscope or a camera, is employed as the detector the pipe must be flushed out of oil in order that optical images of the internal surface of the pipe can be obtained. This requirement is time consuming and results in an interruption in oil extraction, which is both inconvenient and expensive. Damage in the extraction pipe often occur at about sea level but they can also occur below. If long lengths or the whole of the extraction pipe are to be inspected, which may pass several kilometers or more beneath the surface, the cost of an internal inspection may become prohibitively expensive.
It is desirable to find a method in which the inside surface wall of the pipe can be examined without the need to flush the oil from the pipe first.
One advantage of a sonar probe is its ability to derive an image of an object or surface irrespective of whether or not the medium between the probe array sensors and the object or surface under inspection is transparent to light. An appropriate sound speed can be used in the calculation of the waveform generation and processing to compensate for the different speed of sound of different medium. However, using known sonar probes for this purpose give rise to technical difficulties associated with adequate image resolution, these difficulties arising primarily because of the “near-field” imaging effects encountered in confined spaces.
At short distances from a transmit array, the individual parts of the array have significantly different path lengths and as a consequence create an irregular pattern of intensity. This region is known as the ‘near’ field. As the range is increased the differences in the path length across the array gradually reduce to the point where they eventually become insignificant and the energy is considered to emanate from a point source. This region is referred to as the ‘far’ field.
For a receive array a similar effect occurs whereby echoes from objects in the far field can be considered to produce planar wave fronts whereas those from objects in the near field are significantly spherical.
In both cases the transition range between the ‘near’ and ‘far’ fields is determined from the array aperture and the acoustic wavelength at the operating frequency in the propagation medium as follows:Transition range={(Length of acoustic aperture)2×π}/{4×wavelength}
The array aperture is typically the length of the array, and if the array has a curvature the array aperture can be construed as the length of a straight line between and joining the ends of the array.